I have a Brix PC and need as soundcard as the internal one is not very good. I was looking at either this or the Behringher UCA202.

I take it the Wolfson D3's are long gone now? So that would leave me with only the D03k. Is this still a decent device? I would be running the device via the Brix's optical output (which I am not sure is any good or not).

The sound quality you get from this is comparable to a mid-to-high-tier (depending on the model) smartphone audio chip, which, considering the audio chips in most budget TVs are deplorable, is a stellar upgrade for around €30. Highly recommended for use with a TV.

FiiO's marketing on the D03K says: "using Line Output directly on headphone is not recommended".

Why? Can this damage the headphones? Or are they merely suggesting that for some hungrier heaphones the amp isn't powerful enough?

I've just tested the D03K with a pair of Grado SR225i, and they work just fine. Should I be worried about using the D03K and SR225i for prolonged periods?

It just says that because they might not sound the best that way. The circuit isn't designed to drive that kind of load with low distortion. Whether you can actually hear it or not will depend on the exact headphones, the volume you you listen at, and a lot of other little things.

FiiO's marketing on the D03K says: "using Line Output directly on headphone is not recommended".

Why? Can this damage the headphones? Or are they merely suggesting that for some hungrier heaphones the amp isn't powerful enough?

I've just tested the D03K with a pair of Grado SR225i, and they work just fine. Should I be worried about using the D03K and SR225i for prolonged periods?

They say this because it is a line out and there often isn't any way of controlling the volume. On a low impedance pair of headphones, it could be deafeningly loud depending on what it is plugged into. I used to use this with the optical output of my PC and I used the line out with the E11. The E11 was very sensitive to volume changes which proved that it the output from the line out was pretty loud. I had no way of controlling the volume until I plugged an amplifier in. On lots of devices that I have used with the D03k, the digital output volume from them has been fixed and extremely loud so again, the only way of solving this is to you some kind of headphone amplifier.

It may not be the same if you happen to be able to control the volume on the device that has the digital output.

With my set-up (TV with Coaxial Out), plugging in the SR225i into the D03K yields just about desired sound levels...

It may damage an other, more sensitive headphone, however. Therefore, FiiO warns customers not to use without an amplifier, so that consumers don't hold them responsible for damaged headphones (or damaged hearing, for that matter).

To be frank, you can order a fantastic Topping NX1 headphone amplifier for around $35, or less, so there's no reason to use this TVdac without an amplifier; gives you that nice element of control, as well as a more well-rounded sound.